The present invention relates to a pressurized dispensing container with a barrier piston between the dispensed material and the propellant and particularly relates to surface treating the permeable portions of the piston to resist penetration of hydrocarbons, compressed gases and the like propellants.
One form of pressurized container used for dispensing fluent material is a barrier or piston container which includes a dispensing nozzle at one end of the container, a barrier piston in the container separating the fluent, usually viscous dispensed material, which is at the valve side of the piston and a liquefied or gaseous propellant under pressure which is on the other side of the piston and which urges the piston toward the valve with sufficient force that when the valve is manually opened, the fluent material is forced through the valve by the propellant pressure applied to the piston.
A barrier may be any type of separating means within the container. One preferred barrier is a rigid piston which is supported and oriented in the container by the container wall and which is moved up the container wall toward the nozzle by the propellant pressure.
Cocking or tilting of the piston in the container should be prevented, since it will block expulsion of all of the dispensable material.
U.S. Pat. No. 4,234,108 discloses a barrier piston container, wherein the barrier piston is sealed to the internal wall of the container by a collar on the piston and by a flange from the collar which engages the container wall and wherein the piston is additionally prevented from cocking or tilting within the container by the presence of an anti-cocking flange around the bottom of the piston. These flanges are integral with the piston and slide along the wall of the container and are the contact regions between the piston and the container. In order to maintain the seal as the piston moves and to accommodate any slight deformities, dents, etc. in the can shape, the sealing flanges in particular are made of a low or medium density polyethylene plastic material or other suitable resilient plastic material. Low density polyethylene gives both the collar and the flange the flexibility required to allow them to be initially compressed radially when the piston is inserted into the can, e.g., through a reduced diameter neck of the can, and gives them the resiliency to cause the collar and flange to expand to their original cross sections so that they can seal in contact with the wall of the container after insertion and throughout movement of the piston along the container. There are slight changes in the diameter of the container due to changes in pressure in the can, e.g., as the piston moves and the propellant fills a greater volume chamber, the temperature to which the can is exposed, etc. Further, the material of the flanges compensates for dents or deformities by the material flexing around major dents and even by the flanges exerting sufficient force to at least partially reform the can and straighten dents, especially if the can is built of thin-walled material.
A flexible, polyethylene material collar and flange combination initially establishes and maintains the required seal. But polyethylene is somewhat permeable to air, nitrogen, argon, carbon dioxide, nitrous oxide and other compressed gases used as propellants and is very permeable to hydrocarbon propellants and to hydrocarbons in general because polyethylene is itself a hydrocarbon.
Substituting a piston with collar and flange made from less flexible materials, including ABS plastic, nylon, polypropylene and similar plastics unfortunately gives the collar and flange combination insufficient flexibility to permit its initial contraction for insertion and its resilient expansion in order to seal in the can. If the piston is forced through a reduced diameter neck of a can, or is even inserted before the neck has been reduced, and if the flange and collar of the piston are reduced and their diameter is decreased by the wall of the can, then in order to ensure the seal with the can wall, the stiffness of the piston and flange makes sliding along the wall difficult due to wall irregularities. The piston may become stalled and not move along the full height of the container, so that the entire contents are not dispensed. As a result, the low density polyethylene or other suitable flexible plastic material should be used. But they have the permeability problem with respect to the propellants used.
When the flange and collar of the barrier piston are too permeable to the gaseous propellant material, it has caused problems. A flange and collar type piston of polyethylene generally cannot be used for dispensing shave gels and gel mousses, popular products normally dispensed by a barrier piston container, because of the permeation and the inherent qualities of the dispensed materials. Further, post-foam gels in particular, and aerosol barrier cans in general use much less VOC'S because only a few grains of liquefied propellant are used, and in many cases, only air is used to dispense products. Pistons for dispensing post-foam gels are made from ABS (acridonitrile butadiene styrene). Although nylon can be used, it is much more expensive. ABS plastic is too stiff to provide a flexible collar and flange combination on the piston. Therefore, a simple cylindrical piston which is smaller in diameter than the container is used and a layer of the dispensed produced itself forms the seal between the piston and the container wall. That type of seal is difficult to make, is also permeable and is subject to bypass when the container is not completely round or has become distorted or when insufficient material has been forced into the annular space between the piston and the container wall, which happens frequently in actual practice. The undesirable bypass and permeation causes the dispensed product to become aerated and to sputter and shoot out of the valve and also cause the product to leak past the piston and enter the propellant chamber below the piston.
When compressed gas is used as a propellant, as is done with caulks, cheeses and similar water-based products, the propellant gas permeates and destroys the quality of the product. In the case of nonwater-based products, such as greases, gas permeation is reduced. However, shelf life of the container filled with the material dispensed is reduced due to the residual gas permeation of the material.